Silence various warnings in bootstrap build.
auto merge of #7876 : pnkfelix/rust/fsk-silence-warnings-from-bootstrap-build, r=alexcrichton

r? anyone

The only bit that I'm a little concerned about is whether there's some way the assignments to `hi` could somehow still be necessary; but I think that could only be the case if it had been `&const` borrowed (or whatever the hypothetical syntax is for that), and that's not going on in this file.